1. Field of the Invention
The present invention relates generally to a visual inspection apparatus and method, and more particularly to a visual inspection apparatus and method applied to find defects in patterns on a semiconductor wafer, a photo mask, a liquid crystal display, or the like.
2. Description of Related Art
Conventionally, two adjacent chips are compared to each other in order to inspect patterns on a semiconductor wafer, a photo mask, a liquid crystal display, or the like. In order to inspect the patterns, an image obtaining part, which is composed of an optical microscope and an imaging device such as a time delay and integration (TDI) sensor, obtains images of the patterns represented with multiple values while continuously scanning the object along the X-axis. The obtained images are stored in an image data storage part such as a memory. When two images in the corresponding areas on the adjacent first and second chips are obtained, sub-pixel alignment is performed for these two images at regular frame intervals, and the two images are compared with each other on a pixel-by-pixel basis. In this comparison, a pair of a pixel of the first image and the corresponding pixel of the second image that has a gray level difference in excess of a preset threshold is recognized as having a possibility of being defective. At this point in such single detection, it is not clear which chip of the first and second chips has a possibility of being defective, and thus, a differential image of the first and second images is temporarily stored in a defect detecting part as two values. The above-mentioned comparison is performed between the second chip and the third chip to obtain another differential image, which is collated with the differential image of the first and second chips. It is therefore possible to determine which chip of the first and second chips has a possibility of being defective. In this detecting method (double detection), it is possible to determine defective parts on the chips and improve the reliability of the results since the same chip is subjected to the comparison twice.
In the conventional method, however, the chips except end chips (peripheral chips) in the same row or column are compared with the adjacent two chips in the same scanning to thereby accurately detect the defects. Since there are no chips outside the peripheral chips, only the single detection can be performed for the peripheral chips. Thus, the unreliable inspection is performed, or the peripheral chips are not inspected at all.
There is a conventional image comparison method, which comprises the steps of rescanning only the peripheral chips, which have been detected as having a possibility of being defective in the single detection, and comparing those peripheral chips with the second chips to the inside from those peripheral chips in order to find whether the peripheral chips have a possibility of being defective. This method, however, is inefficient since it is necessary to rescan the peripheral chips. In order to eliminate the need for rescanning the peripheral chips, the images of the peripheral chips and the second chips to the inside from the peripheral chips are stored in an image storage part such as a memory, and the images of these chips are read from the image storage part on completion of the scanning to compare the images. This is also inefficient since the third chip must be compared three times with the first, second and fourth chips. An ordinary comparison unit has only the minimum capacity for processing the captured images within a set amount of time, and therefore, the extra comparison lowers the inspection speed. To solve this problem, only the image of the first chip may be stored in the memory until the completion of one scanning, and the images of the peripheral chips stored in the previous scanning are compared with one another while the image of the first chip is captured in the next scanning, thereby performing the double detection for all the chips. If the distance between the peripheral chips becomes longer with the increase in the diameters of the wafers, a slight variation in a manufacturing process will cause noise problems such as odd color. This deteriorates the inspection sensitivity. It is therefore desirable to compare the closest chips available.
Alternatively, in a scanning method of Japanese Patent Provisional Publication No. 2-210249, the last chip in each scanning row or column is compared with the first chip in the next scanning row or column to perform the double detection even for the peripheral chips. Since, however, the straightness in the scanning direction (the X-axis) is higher than the absolute position accuracy along the Y-axis, the difference of the images along the Y-axis is greater than that of the images of chips in the same scanning rows. To correct the difference of the images, the images are normally shifted by less than a pixel and laid upon one another (a sub-pixel alignment). In this case, however, there is always such a possibility that there is a difference of 0.5 pixel at the maximum. The more the difference is corrected in the sub-pixel alignment, the lower the defect defecting sensitivity becomes due to the deterioration of the captured images. Moreover, this method cannot be applied to the inspection for only one row or column.
In view of the foregoing, it is an object of the present invention to provide a visual inspection apparatus, which is capable of detecting defects efficiently and accurately for all the chips including peripheral chips in double detection by comparing images a minimum amount of times.
To achieve the above-mentioned object, the present invention is directed to a visual inspection apparatus, comprising: an imaging means for capturing images of at least three areas arranged in a line on an object; an image comparison means for dividing the areas into at least one group, each group including at least consecutive three of the areas; for designating one of the areas as a subject area and other two of the areas as comparison areas for the subject area, the comparison areas being in the same group with the subject area and within a predetermined distance from the subject area; and for comparing the image of the subject area with the images of the comparison areas; and a defect detection means for detecting a defect in the object in accordance with the comparison between the images of the areas by the image comparison means.
The visual inspection apparatus is preferably characterized in that: the image comparison means numbers the areas in each group along the line; if the subject area is odd-numbered and is not one of ends of odd-numbered areas in the group, the comparison areas are two odd-numbered areas closest to the subject area; if the subject area is one of the ends of the odd-numbered areas in the group, the comparison areas are one odd-numbered area and one even-numbered area closest to the subject area; if the subject area is even-numbered and is not one of ends of even-numbered areas in the group, the comparison areas are two even-numbered areas closest to the subject area; and if the subject area is one of the ends of even-numbered areas in the group, the comparison areas are one even-numbered area and one odd-numbered area closest to the subject area.
Preferably, the imaging means relatively scans the object along the line by one of a line sensor and a TDI sensor to sequentially capture the images of the areas.
The visual inspection apparatus of the present invention scans the object along the X-axis to sequentially capture the images of the areas with the same pattern, which are arranged along the X-axis. Then, the areas arranged along the X-axis are divided into a predetermined number of groups. In each group, two areas in a predetermined interval are respectively combined with one another so that each area can be combined with other two areas. The image of each area, which is captured by the imaging means, is compared with the images of the other two areas. It is therefore possible to compare the images of all the areas including both ends (the peripheral chips) in the double detection in the minimum amount of comparison times (the same as the number of areas arranged along the X-axis). In addition, the image of each area is compared to the images of the close areas, and thus, the defects can be detected accurately.
To achieve the above-mentioned object, the present invention is directed to a visual inspection method, comprising the steps of: capturing images of at least three areas arranged in a line on an object; dividing the areas into at least one group, each group including at least consecutive three of the areas; designating one of the areas as a subject area and other two of the areas as comparison areas for the subject area, the comparison areas being in the same group with the subject area and within a predetermined distance from the subject area; and comparing the image of the subject area with the images of the comparison areas to determine whether the subject area is defective.
The visual inspection method is preferably characterized in that: the designating step comprises the step of numbering the areas in each group along the line; if the subject area is odd-numbered and is not one of ends of odd-numbered areas in the group, the comparison areas are two odd-numbered areas closest to the subject area; if the subject area is one of the ends of the odd-numbered areas in the group, the comparison areas are one odd-numbered area and one even-numbered area closest to the subject area; if the subject area is even-numbered and is not one of ends of even-numbered areas in the group, the comparison areas are two even-numbered areas closest to the subject area; and if the subject area is one of the ends of even-numbered areas in the group, the comparison areas are one even-numbered area and one odd-numbered area closest to the subject area.